1. If "0" is not a string value, try removing ""?
2. If you're using / try setting it as MM/DD/YYYY format - I think it works better.
Hope this helps.
Have your tried using single quotes around your arguments for year month and day e.g. 'MID([Original Call Date Key],2,2)'
Thanks everyone for the help but I found a way to do it,
IF [Date Key] = 0 THEN NULL
LEFT(STR([Date Key]),4) + "-" + MID(str([Date Key]),2,2) + "-" + RIGHT(str([Date Key]),2)
That's where I was approaching - if you're prone to use "/" then change the order, YYYY MM DD order only works well with "-"